Determining Shear - Wave Velocity from Diffracted S H Waves

Profiles of SH waves diffracted around the core (Sd) for three deep events at stations across North America and the Atlantic (4 = 92 ° to 152 °) are used to determine the properties of the lower mantle in the vicinity of the core-manUe boundary (CMB). The S-wave velocity above the CMB is found to be tic = 7.22 __ 0.1 km/sec, in agreement with gross earth models, but higher than previously reported values from direct measurements of Sd. The frequency imdependence of the Sd ray parameter argues strongly against the possibility of a low-velocity zone immediately above the core mantle boundary. We compute synthetic seismograms for Sd by summing normal modes. A comparison of the present data with a synthetic profile for earth model 1066A gives excellent agreement at periods greater than 45 seconds. Synthetics for other models are used to substantially constrain the possibility of significant rigidity of the uppermost layer of the core. INTRODUCTION The seismic properties of the deepest parts of the mantle, in the vicinity of the core-mantle boundary (CMB) have been the subject of extensive, and at times controversial, studies. Individual observations of a decrease in S-wave velocity near the CMB (Cleary, 1969; Bolt et al., 1970) and of an associated low-Q zone (Mikumo and Kurita, 1968) have been reported. Some data obtained from free oscillation Q studies (Anderson and Hart, 1978) have supported these reports, although gross earth models (Gilbert and Dziewonski, 1975; Anderson and Hart, 1976) have usually failed to yield a low shear velocity zone at the base of the mantle. Also, an increase in S-wave velocity above the CMB has been proposed by Mitchell and Hehnberger (1973) on the basis of Scs to S amplitude ratios. The seismic properties just above the CMB have significant geophysical implications. A low-Q, low-S velocity zone might be related to efficient heat transfer across the boundary, and possibly, also, to a difference in chemical content in the deepest shells of the mantle. In turn, the composit ion of the deepest mant le bears directly on our unders tanding of the differentiation process in the Ea r th and other planets (Jacobs, 1975). Most of the seismic evidence for a low-S wave velocity zone at the base of the mant le comes from the s tudy of S H waves diffracted along the CMB into the shadow zone for direct S waves. We will call this phase Sd. Cleary et al. (1967), Cleary (1969), Bolt et al. (1970), and Hales and Rober ts (1970) repor ted values obtained from Sd studies of 7.06, 6.8, 6.99, and 6.78 km/sec, respectively, for the S-wave velocity at the base of the mantle, tic, as opposed to Jeffreys ' 7.30, Anderson and Har t ' s 7.23, or Gilbert and Dziewonski's 7.25. Mondt (1977), using the techniques of Scholte (1956), Richards (1970), and Chapman and Phinney (1972), suggested a value of fie = 7.03 km/sec from the ampli tude decay of Sd waves with distance. The pr imary purpose of this paper is to resolve the question of the S velocity at the base of the mantle, by using a set of three profiles of high-quality Sd data, * Present address: Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520. t Present address: Department of Geophysics, Stanford University, Stanford, California 94305. 1039 1040 E M I L E A . O K A L A N D R O B E R T J . G E L L E R sampling the CMB in two different geographical areas. By measuring d T / d A of S d using two different methods, we obtain an S-wave velocity fl~ = 7.22 _ 0.1 km/sec immediately above the CMB. As this result rests on geometrical optics, we conduct some synthetic seismogram experiments at long periods, using normal mode summation, to confirm this result. The synthetic seismograms for Gilbert and Dziewonski's (1975) model 1066A agree very well with the observed profiles. In contrast, observed amplitudes for S d at large distances (A > 120 °) are inconsistent with models including a small but finite (fi = 0.73 2 km/sec) rigidity in the core, in a layer extending 25 km or more below the CMB. The existence of S d was mentioned by Gutenberg and Richter (1935), but the first reported data are found in Lehmann (1953). Cleary et al. (1967) first measured the apparent slowness of the phase. Their value (p = 8.61 sec/deg) corresponds to fi~ = 7.06 km/sec for a radius of the core, rc = 3485 km, through the relation